At present, metal sheets such as electroinplate, tin free steel and aluminum sheet are widely used for can stock after coating with lacquer one or several times. The lacquer coating is disadvantageous from the standpoint of energy cost because a long time is required for curing the lacquer and a large volume of solvent discharged during lacquer curing must be burned in another furnace in order to prevent air pollution.
Recently, the lamination of thermoplastic resin film on a metal sheet was attempted in order to avoid these problems. For example, the methods shown in Japanese Patent Publication No. Sho 60-47103, Laid--Open Japanese Patent Application No. Sho 61-149340, Japanese Patent Publication No. Sho 63-13829, Laid-Open Japanese Patent Application No. Sho 61-149341 and Japanese Patent Publication No. Sho 59-46786 are already known.
Japanese Patent Publication No. Sho 60-47103 and Laid-Open Japanese Patent Application No. Sho 61-149340 relate to a process for lamination of a crystalline polyester resin film to a metal sheet by heating the sheet to above the melting point of said polyester resin film and thereafter immediately quenching the laminate. In these patents, the crystalline polyester resin film is sufficiently adhered to the metal sheet by an amorphous non-oriented polyester resin layer which is formed at the interface of the crystalline polyester resin film and the metal sheet as a result of the heating step. However, when the polyester resin film laminated metal sheet according to said patents is reheated to the temperature of 160.degree. to 210.degree. C. for 10 to 30 minutes required for curing the printing ink or lacquer coated on said metal sheet before forming, adhesion of the polyester resin film becomes noticeably poor. Furthermore, when the polyester resin film laminated metal sheet according to the above cited patents is treated by hot steam at 100.degree. to 135.degree. C. in a retort, the surface of the polyester resin film laminated metal sheet becomes milky. Especially, the part of the surface of said polyester resin film laminated metal sheet, where the hot water droplets pile up, becomes noticeably milky. The reasons for the deterioration of adhesion strength and the discoloration of the surface appearance described above are due to the recrystallization of the amorphous non-oriented polyester resin layer by reheating or by contact with hot steam.
On the other hand, the surface of the polyester resin film laminated metal sheet according to Japanese Patent Publication No. Sho 63-13829 and Laid-Open Japanese Patent Application No. Sho 61-149341 does not become milky by contact with hot steam in a retort, because an amorphous non-oriented polyester resin layer is not present as a result of lamination at a temperature below the melting point of said polyester resin film. However, the coating and curing of the specified adhesive which is indispensable in said patents is disadvantageous from the standpoint of production cost.
In Japanese Patent Publication No. Sho 59-46786, there is disclosed a process for laminating an oriented polyester sheet to metal by a simultaneous insertion of said polyester sheet and metal between a pair of rolls which is heated to a temperature below the melting point of said polyester sheet. However, the adhesion strength of the polyester sheet to the metal is remarkably poor as shown in the examples in the above patent because the polyester sheet is directly laminated to an untreated metal surface. Therefore, the laminate according to that patent is not applications where excellent adhesion of the laminated polyester film to a metal sheet is required even after forming i.e. to deep drawn cans and can ends.
Accordingly, it is a first objective of the present invention to provide a polyester resin film laminated metal sheet having excellent heat resistance wherein the laminated polyester resin film is not peeled off from the surface of the metal by forming even after reheating before forming and the surface appearance does not change after treatment by hot steam in a retort.
It is the second objective of the present invention to provide a method for the continuous lamination at high speed of polyester resin film on one or both sides of a metal sheet.